1. Field
Embodiments of the present invention generally relate to the detection of materials, including hazardous and explosive materials.
2. Related Art
Detection of materials, such as energetic or explosive materials, toxic industrial chemicals, and chemical agents, is of great importance for various military and homeland security applications. Energetic materials may include 1,3,5-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazoisowurtzitane (CL20), for instance.
Conventional methods for detection of energetic materials, however, pose a challenge because these materials typically have very low vapor pressures and present a minimal amount of material to conventional detectors that sense vapors. As such, these materials may be difficult to detect spectroscopically at ambient conditions. The difficulty stems in part because they do not emit sufficient vapors at atmospheric pressure for their sensing. For example, RDX has a vapor pressure of about 5×10−9 torr (app. 7×10−7 Pascal) or 7 ppt/v (parts per trillion by volume) at ambient conditions, which is about three orders of magnitude less than TNT. Other hazardous compounds, such as some chemical nerve agents, e.g. VX, also have low vapor pressures at ambient conditions, and thus present a similar challenge for their detection. An added complication is that vapor molecules of energetic materials and other materials tend to readily adsorb to surfaces, so that molecules available for detection are reduced further. Material detection by light emission and/or absorption is challenging because the resulting spectra lack distinguishable features, particularly in the ultraviolet and visible region of the electromagnetic spectrum, the emissions signals are quenched by collisions at atmospheric pressure, or a combination thereof.
Numerous methods and devices have been developed to reduce the risk to the general population and military personnel by detecting the presence of materials. For instance, ion mobility spectrometry (IMS) with detection limits on the order of 1-10 ppb is the current state of the art for sensing vapors and is commonly in use in airports and other secured areas. This detection limit is still higher than the vapor pressures of many relevant energetic materials.
Laser-based strategies also exist for detecting energetic materials on surfaces. Some laser spectroscopic strategies include laser induced breakdown spectroscopy (LIBS), laser photofragmentation—fragment detection (PF-FD), and Raman spectroscopy. LIBS uses high-powered, focused laser beams to completely break down a complex energetic material into its constituent elements. When using LIBS as a detection technique, the breakdown of energetic material into atomic constituents can hinder positive energetic material identification. In contrast, PF-FD breaks down the complex explosive molecule into larger fragments or signature molecular groups such as NO2 and NO that are subsequently detected by UV laser induced fluorescence (LIF) and/or resonance-enhanced multiphoton ionization (REMPI), thus indicating the presence of the energetic material. Both LIBS and PF-FD are indirect methods for energetic materials detection. They do not identify the energetic materials, but instead identify characteristic fragments resulting from the photolysis or decomposition of the energetic materials. As a result, the selectivity of these methods is not as high as those that involve direct detection. Also, trace quantities of atmospheric nitrous oxide (NO2) may interfere with the measurements. This potentially leads to high false positive rates.
Raman spectroscopy is a laser-based approach that has proven successful to detect directly energetic materials. The technique uses a laser to probe the molecule's fundamental vibrational and rotational states from the inelastic scatter of photons. However, without UV resonance enhancement, Raman spectroscopy suffers from weak signals. Furthermore, practical Raman instruments require high resolution dispersive elements and an arrayed charge-coupled device (CCD), or complementary metal-oxide-semiconductor (CMOS) detector arrays, resulting in complex electronics. Additional electronics lead to high complexity and power consumption.
Photoacoustic spectroscopy may be used for detecting energetic materials. Alexander Graham Bell discovered the photoacoustic effect in 1881, when he found that materials emit sound when exposed to a rapidly interrupted beam of sunlight. The sample converts part of the absorbed laser light into heat, which can be transferred to the ambient air. The pressure fluctuation caused by rapid sample heating and cooling appears as compression and rarefaction of the air and results in sound. Absorption features in the ultraviolet, visible, or infrared region of the electromagnetic spectrum correspond to a molecule's electronic (ultraviolet and visible) or vibrational transitions (infrared). Most molecules contain broad spectral features in the ultraviolet region, but exhibit sharp, well-defined features in the infrared. These defined features enhance a sensor's selectivity and, as a result, many of them employ infrared wavelengths.
Ethylene glycol dinitrate (EGDN), nitroglycerine (NG) and 2,4-Dinitrotoluene (DNT) vapors have reportedly been detected in the parts per billion using a 9.6 μm CO2 laser, and spectra and models of TNT and RDX at CO2 laser wavelengths ranging from 9.6 to 11.6 μm have been reported. Recently, a photoacoustic spectroscopy system and technique for remote sensing of explosives toxic chemicals was disclosed using a pulsed tunable laser such as a CO2 laser to detect NG, DNT, TNT, and ammonium nitrate (NH4NO3). However, the low vapor pressure of most explosives, such as RDX and CL20, precludes their detection with this technique at ambient temperature and pressure. In the case of TNT, the sensor's signal-to-noise ratio is poor because the vapor pressure of TNT is low at ambient conditions. Also, ambient levels of CO2, NH3, O3, and H2O interfere strongly in this spectral region.
And, more recently, remote identification of gas-phase explosives and other harmful materials by semiconducting nanoparticle photoluminescence or photoacoustics have been reported. As mentioned above, though, gas-phase detection of most explosives cannot be achieved at room temperature and pressure, and for limited cases such as TNT, the signal-to-noise is very poor. Nanoparticle luminescence is restricted to energetic materials synthesized with such particles. Most energetic materials do not contain these particles because they may lower the overall system performance or because they may help in tracking the energetic materials' origin.
Energetic materials possess vibrational absorption features which can be exploited for detecting many compounds in the vapor phase. The use of tunable cascade lasers and photoacoustics to detect trace gases, TNT, triacetone triperoxide (TATP), and precursors of acetone and hydrogen peroxide have been disclosed. While this technique may work for samples with high vapor pressures at ambient conditions, it has been shown to work very poorly for various energetic materials such as TNT because it has very little vapor pressure at ambient conditions. Solid-phase RDX and CL20 have vapor pressures that are several orders of magnitude lower than TNT at ambient conditions and they will present a formidable challenge for this technique.
Improved detection of materials would be useful.